White light emitting LEDs (“white LEDs”) are known in the art and are a relatively recent innovation. Due to their long operating life expectancy (>50,000 hours) and high luminous efficacy (70 lumens per watt and higher) high brightness white LEDs are increasingly being used to replace conventional fluorescent, compact fluorescent and incandescent light sources.
It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught for example in U.S. Pat. No. 5,998,925, white LEDs include one or more photoluminescent materials (e.g., phosphor materials), which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light, green and orange or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor combined with the light emitted by the phosphor provides light which appears to the human eye as being nearly white in color.
The exact color generated by the LED light is highly dependent upon the amount of light that is emitted by the phosphor material, since it is the combination of the amount (and wavelength) of the phosphor-emitted light and the amount (and wavelength) of the residual blue light that determines the color of the resultant light. Therefore, phosphor-based LED devices that are intended to generate white light will require sufficient amounts of phosphors to function correctly, since the phosphor-based LED device having insufficient amounts of phosphor materials will fail to generate white-appearing light.
The problem is that phosphor materials are relatively costly, and hence correspond to a significant portion of the costs for producing phosphor-based LED devices. Typically the phosphor material in a LED light is mixed with a light transmissive material such as a silicone or epoxy material and the mixture directly applied to the light emitting surface of the LED die. This results in a small footprint layer of phosphor materials placed directly on the LED die, that is nevertheless still costly to produce in part because of the significant costs of the phosphor materials.
As disclosed in United States patent application 2008/0218992 A1 to Li, it is also known to provide the phosphor material as a layer on, or incorporate the phosphor material within an, optical component that is physically located remote to the LED die. This typically results in a layer of phosphor materials having a much larger footprint than the approach described in the preceding paragraph. Because of its larger size, a much greater amount of phosphor is normally required to manufacture such “remote phosphor” LED devices. As a result, the costs are correspondingly greater as well to provide the increased amount of phosphor materials needed for such remote phosphor LED devices. For example, U.S. Pat. No. 7,937,865 teaches solid-state light emitting signs in which blue light from an LED is used to excite phosphor materials on a light emitting signage surface to generate a desired color of light. A large quantity of the phosphor materials must normally be present to populate the expanse of the light emitting signage surface for the device to produce the appropriate color for its intended light functionality.
Therefore, there is a need for improved approach to implement LED lighting apparatuses that maintains the desired color properties of the devices, but without requiring the large quantities of photo-luminescent materials (e.g. phosphor materials) that are required in the prior approaches.
It is an object of some embodiments of the present invention to provide a light emitting device, a light emitting sign, a photoluminescence wavelength conversion component and a photoluminescence signage surface that in part at least overcomes the limitations of the known devices.